Dr. Paul Offit, a professor at the University of Pennsylvania and a member of the F.D.A.s vaccine advisory panel, said that pausing a trial is a huge logistical challenge especially for one like Johnson & Johnsons, with plans for 60,000 volunteers in 10 countries.

Its this big warship that you just stop moving, Dr. Offit said.

Once a trial is paused, a safety board may ask for a volunteer who experienced an adverse event to be unblinded in other words, to find out if the volunteer got the placebo or the treatment. If the volunteer received a placebo, then the treatment cant be the cause of the event and the trial can continue.

If it turns out that the volunteer got the treatment, the board does a flurry of detective work. The members look over the medical records. They may ask for more information about volunteers health or even order new tests not just for the people who experienced adverse events, but for everyone in the trial.

The board uses this evidence to come to a conclusion about whether the treatment most likely had anything to do with the event. On very rare occasions, for example, some vaccines can cause a nerve disorder called Guillain-Barr syndrome. But the condition takes weeks to develop. If a volunteer shows signs of Guillain-Barr syndrome on the day of a vaccine injection, it cant be the cause.

Regulators then review the decision of these boards and may accept it or ask for more information. For trials that are running in several countries at once, this review can make pausing a trial even more of a challenge. After AstraZeneca paused its global trials on Sept. 6 for a review, regulators in Brazil, India, Japan, South Africa and the United Kingdom all gave the green light for the trial to resume. But American regulators are still keeping the U.S. trial on pause as they continue to look over the evidence.

If a safety board rules that an adverse event most likely was not a result of the vaccine or treatment, it may allow the trial to start up again. If, on the other hand, theres some urgent problem a contaminated batch of drugs, for example the trial may have to stop. When the evidence isnt so clear, the board may let the trial resume with extra tests or exams. A second case of the same event might be more common than you would expect from chance, forcing the trial to end.

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3 Covid-19 Trials Have Been Paused for Safety. Thats a Good Thing. - The New York Times